1. Field of the Invention
The invention relates to an X-ray examination apparatus, comprising a X-ray source and an X-ray detector wherebetween there is arranged a filter which comprises a plurality of filter elements having an X-ray absorptivity which can be adjusted by controlling a quantity of X-ray absorbing liquid within individual filter elements.
2. Description of the Related Art
An X-ray examination apparatus of this kind is known from French Patent Application FR 2 599 886. The known X-ray apparatus comprises a filter for limiting the dynamic range of an X-ray image, being the interval between the extremes of the brightness values. An X-ray image is formed on the X-ray detector by arranging an object, for example a patient to be examined, between the X-ray source and the X-ray detector and by irradiating said object by means of X-rays emitted by the X-ray source. If no steps are taken, the dynamic range of the X-ray image may be large. On the one hand, for some parts of the object, for example lung tissue, the X-ray transmittance will be high whereas other parts of the object, for example bone tissue, can hardly be penetrated by X-rays. Lead shutters which are used to intercept pans of the X-ray beam emitted by the X-ray source in order to shield pans of the object to be examined from the X-rays are imaged with a uniform, very low brightness. Lead shutters are also used to prevent X-rays which do not pass through the object from reaching the X-ray detector, thus causing overexposures in the X-ray image. If no further steps are taken, therefore, an X-ray image is obtained with a large dynamic range whereas, for example medically relevant information in the X-ray image is contained in brightness variations in a much smaller dynamic range; because it is practically impossible to make small details of low contrast suitably visible in a rendition of such an X-ray image, such an X-ray image cannot be used very well for making a diagnosis. Furthermore, problems are encountered when such an X-ray image is picked up by means of an image intensifier pick-up chain. An image intensifier pick-up chain comprises an image intensifier tube for convening an incident X-ray image into a light image and a video camera for deriving an electronic image signal from the light image. From regions of very high or very low brightness in the X-ray image, regions of very high and very low brightness, respectively, are formed in the light image. If no further steps are taken, the dynamic range of the light image could be larger than the range of brightness values that can be handled by the video camera without causing disturbances in the electronic image signal.
In order to limit the dynamic range of the X-ray image, the known X-ray examination apparatus comprises a filter with filter elements provided with a bundle of parallel capillary tubes, each of which is connected, via a valve, to a reservoir containing an X-ray absorbing liquid which suitably wets the inner walls of the capillary tubes. In order to fill a capillary tube with the X-ray absorbing liquid, the valve of the relevant capillary tube is opened, after which the capillary tube is filled with the X-ray absorbing liquid by the capillary effect. Such a filled capillary tube has a high X-ray absorptivity for X-rays passing through such a filled capillary tube in a direction approximately parallel to its longitudinal direction. The valves are controlled so as to ensure that the amount of X-ray absorbing liquid in the capillary tubes is adjusted so that in parts of the X-ray beam which pass through parts of low absorptivity of the object filter elements are adjusted to a high X-ray absorptivity and that filter elements in parts of the X-ray beam which pass through parts of high absorptivity of the object, or are intercepted by a lead shutter, are adjusted to a low X-ray absorptivity.
In order to change the adjustment of the filter of the known X-ray examination apparatus it is necessary to empty tilled capillary tubes first. Therefore, use is made of a paramagnetic X-ray absorbing liquid which is removed from the capillary tubes by application of a magnetic field. After all capillary tubes have been emptied, the filter is adjusted anew by de, activation of the magnetic field and by subsequently opening valves of capillary tubes which are filled with the X-ray absorbing liquid for the new filter setting so as to adjust these tubes to a high X-ray absorptivity.
It is a drawback of the known filter that it is practically impossible to change the setting of the filter within a brief period of time, for example one second. Therefore, the known X-ray apparatus is not suitable for forming successive X-ray images at a high image rate when the setting of the filter is changed between the formation of successive X-ray images. Because it is necessary to empty all capillary tubes before the filter elements can be adjusted to new X-ray absorptivities and because the X-ray absorbing liquid suitably wets the inner wall of the capillary tube so that emptying requires a substantial period of time, i.e. several seconds or even tens of seconds, switching over the known filter is rather time-consuming. Moreover, it is not readily possible to make the capillary tube completely empty by application of the magnetic field, because a layer of X-ray absorbing liquid will adhere to the inner walls of the capillary tubes.
It is a further drawback of the known filter that the construction utilizing separate mechanical valves for each of the capillary tubes is rather complex.